THE picturesque village of Sandy Hook in Connecticut is reeling as it struggles to comprehend how it has become the scene of the second worst massacre in US history, after a 20-year-old man shot dead 20 young students and six adults inside a school before killing himself.

Wearing a bullet-proof vest in what appeared to be a planned event, Adam Lanza entered Sandy Hook Elementary School, at around 9.30am on Friday morning, US time, killing his mother, a kindergarten teacher, before turning his automatic weapons on a group of children.

Lanza appears to have targeted just one part of the school, where his mother worked.

As many as 100 shots were heard throughout the school, believed to come from Lanza’s Glock and Sig Sauer automatic pistols, which he appeared to have reloaded during his spree. They were found at his side.

It is feared most of the dead children at the school, which goes up to Grade Four, were kindergarten students.

Connecticut State Police lead children from the Sandy Hook Elementary School. Picture: AP/ Newtown Bee

Lanza's older brother, 24-year-old Ryan, of Hoboken, N.J., was being questioned, but a law enforcement official told AP that Adam Lanza had a possible personality disorder said Ryan Lanza was not believed to have had any role in the rampage.

A younger photo of Adam Lanza, the suspect involved in the shooting of 26 people. Picture: Screen grab from ABCSource: Supplied

Police arrived at the scene around 9.40am after taking a 911 call. Teachers at the school blockaded their students in their classrooms, with one young boy saying he saw bullets flying down a corridor.

Early reports were that the gunman was dead and that three children had been taken to hospital.

Heavily armed Connecticut State troopers are on the scene at the Sandy Hook School following a shooting at the school. Picture: AP

The full horror of the atrocity began to emerge at about 1pm, as reports came out that 18 children were dead. In the end, two more children died at hospital, and six adults were confirmed dead within the school grounds..

Police had not given any motive for the massacre, but locals in Sandy Hook and Newtown were of the opinion that no rational explanation was possible..

The children’s and teacher’s bodies were still in the school a short while ago, as police and worked over what must be an horrific crime scene.

Throughout the day, it was a tragic waiting game as parents and relatives awaited the names of the dead, who were all no doubt looking forward to a peaceful Christmas in the community of 28,000, until now known as one of the safest communities in the US..

Responders gather at the scene of a mass school shooting at Sandy Hook Elementary School. Picture: AFP

"They had their entire lives ahead of them—birthday, weddings, kids of their own," said an emotional President Barack Obama, who suggested that America’s permissive gun culture was to blame..

"We've endured too many of these tragedies in the past few years,” he said. “We are going to have to come together to take meaningful action to prevent more tragedies like this regardless of the politics.".

Jordan Stofko, 12, who spent her first years at the school, wept. "I’m feeling really worried because I don’t know who passed, and if I know them," she told News Ltd.

"My neighbour was in the room and survived it. I’m feeling really sorry for everyone, and the parents."

Residents grieve following a shooting at Sandy Hook Elementary School on December 14, 2012 in Newtown, Connecticut. Picture: AFP

Her father, Chuck, said it was a nightmare. “I heard that the madman had gone in there and killed his mother, and then took out a whole classroom of kindergarteners,” he said.

"I know that we have the prayers and the support of the world."

Monsigneur Robert Weiss, of Newtown’s St Rose church, spent the day consoling parents of the dead children at the firehouse near the school, which became a staging ground for bereft families.

He said a mother broke down as he was talking to her. Her beeper went off, reminding her that she had to take her son, who was dead, to cub scouts.

Parents walk away from the Sandy Hook Elementary School with their children following a shooting at the school. Picture: AP

"I was there when they told a little boy that it was his sister who had passed on,” the priest said. “To watch this little boy’s reaction, which was I’m not going to have anyone to play with now. Eventually he just broke down in tears.

“There was a lot of heavy emotion.”

Police were keeping the families of the victims away from the media, which converged on the heartbroken village in huge numbers.

“It was horrendous,” parent Brenda Lebinski told Reuters, after rushing to the school to check on her daughter, who is in the third grade.

"Everyone was in hysterics - parents, students. There were kids coming out of the school bloodied. I don't know if they were shot, but they were bloodied.”

The Sandy Hook massacre is now second--worst gun massacre in US history after 2007’s Virginia Tech, in which 32 were killed.

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